1. Field of the Invention
The present invention relates to the field of optically pumped atomic clocks or magnetometers, and more particularly to atomic clocks or magnetometers having simultaneous locking of field and frequency with end resonances.
2. Description of the Related Art
Conventional, gas-cell atomic clocks utilize optically pumped alkali-metal vapors. Atomic clocks are utilized in various systems which require extremely accurate frequency measurements. For example, atomic clocks are used in GPS (global positioning system) satellites and other navigation and positioning systems, as well as in cellular phone systems, scientific experiments and military applications.
In one type of atomic clock, a cell containing an active medium, such as rubidium or cesium vapor, is irradiated with both optical and microwave power. The cell contains a few droplets of alkali metal and an inert buffer gas at a fraction of an atmosphere of pressure. Light from the optical source pumps the atoms of the alkali-metal vapor from a ground state to an optically excited state, from which the atoms fall back to the ground state, either by emission of fluorescent light or by quenching collisions with a buffer gas molecule like N2. The wavelength and polarization of the light are chosen to ensure that some ground state sublevels are selectively depopulated, and other sublevels are overpopulated compared to the normal, nearly uniform distribution of atoms between the sublevels. It is also possible to excite the same resonances by modulating the light at the Bohr frequency of the resonance, as first pointed out by Bell and Bloom, W. E. Bell and A. L. Bloom, Phys. Rev. 107, 1559 (1957), hereby incorporated by reference into this application. The redistribution of atoms between the ground-state sublevels changes the transparency of the vapor so a different amount of light passes through the vapor to a photo detector that measures the transmission of the pumping beam, or to photo detectors that measure fluorescent light scattered out of the beam. If an oscillating magnetic field with a frequency equal to one of the Bohr frequencies of the atoms is applied to the vapor, the population imbalances between the ground-state sublevels are eliminated and the transparency of the vapor returns to its unpumped value. The changes in the transparency of the vapor are used to lock a clock or magnetometer to the Bohr frequencies of the alkali-metal atoms.
The Bohr frequency of a gas cell atomic clock is the frequency ν with which the electron spin S processes about the nuclear spin I for an alkali-metal atom in its ground state. The precession is caused by the magnetic hyperfine interaction. Approximate clock frequencies are ν=6.835 GHz for 87Rb and ν=9.193 GHz for 133Cs. Conventionally, clocks have used the “0—0” resonance which is the transition between an upper energy level with azimuthal quantum number m=0 and total angular momentum quantum number ƒ=I+½, and a lower energy level, also with azimuthal quantum number m=0 but with total angular momentum quantum number ƒ=I−½.
For atomic clocks, it is important to maintain the minimum uncertainty, δν, of the resonance frequency ν. The frequency uncertainty is approximately given by the ratio of the resonance linewidth, Δν, to the signal-to-noise ratio, SNR, of the resonance line. That is, δν=Δν/SNR. Clearly, one would like to use resonances with the smallest possible linewidth, Δν, and the largest possible signal-to-noise ratio, SNR.
For miniature atomic clocks it is necessary to increase the density of the alkali-metal vapor to compensate for the smaller physical path length through the vapor. The increased vapor density leads to more rapid collisions between alkali-metal atoms. These collisions are a potent source of resonance line broadening. While an alkali-metal atom can collide millions of times with a buffer-gas molecule, like nitrogen or argon, with no perturbation of the resonance, every collision between alkali-metal atoms interrupts the resonance and broadens the resonance linewidth. The broadening mechanism is “spin exchange,” the exchange of electron spins within a pair of alkali-metal atoms during a collision. The spin-exchange broadening puts fundamental limits on how small such clocks can be. Smaller clocks require larger vapor densities to ensure that the pumping light is absorbed in a shorter path length. The higher atomic density leads to larger spin-exchange broadening of the resonance lines, and makes the resonance lines less suitable for locking a clock frequency or a magnetometer frequency.
U.S. Pat. No. 2,951,992 describes an atomic frequency standard having a pair of cells of alkali metal vapor in which a substantially homogenous static magnetic field permeates both cells and energy of a sum frequency of a frequency source and an interpolation generator is applied to one cell to excite hyperfine ground energy level transitions therein, and energy of a difference frequency of same frequency source and same interpolation generator is applied to the other of the cells to excite microwave hyperfine energy level transitions in the other cell.
It is desirable to provide a method and system for using end resonances for providing simultaneous locking of field and frequency in the same cell in order to eliminate most of the sensitivity to field differences between the two cells, and to operate atomic clocks at much higher densities of alkali-metal atoms than conventional systems.